This invention relates to an optical lens and a method of producing the same, a method of producing an optical lens array, a focus error signal production method, and an optical pickup apparatus, and more particularly to an optical lens for use with an optical system of an optical disk apparatus or the like, a method of producing the same and a method of producing an optical lens array for use with an optical system of an optical disk apparatus or the like, a focus error signal production method for an optical disk apparatus, and an optical pickup apparatus suitable for use with an optical disk apparatus.
In recent years, in the field of information recording, much effort has been and is being directed to investigations for an optical information recording system. The optical information recording system is advantageous at least in that it allows recording and playback without contact, that it can achieve a recording density higher by one or more digit than that by a magnetic recording system, and that it can be applied to various memory types including the reproduction-only type, the write-once type and the erasable type. Therefore, the optical information recording system is expected as a system which allows realization of a less expensive file of a large capacity and can be used for various applications from industrial applications to consumer applications.
In an optical pickup apparatus incorporated in a recording and/or playback apparatus (hereinafter referred to sometimes as optical disk apparatus) for an optical recording medium (hereinafter referred to sometimes as optical disk) such as a compact disk (CD) or a digital versatile disk (DVD) for such various optical information recording systems as described above, laser light from a laser diode which emits a laser beam of a wavelength of, for example, 780 nm or 650 nm is condensed on an optical recording layer of an optical disk by an optical system including an optical member such as a beam splitter. Reflected light from the optical disk follows a reverse route through the optical system and is projected on a light receiving element such as a photodiode by a multi-lens device.
Information recorded on the optical recording layer of the optical disk is thus read out depending upon the variation of the reflected light from the optical disk.
In the optical pickup apparatus described above, since the distance between an objective lens for condensing the laser light upon the optical recording layer of the optical disk and the optical disk has an influence on the diameter of a spot of the laser light on the optical recording layer, the relative position of the objective lens to the optical disk is adjusted by a focusing servo mechanism.
The focusing servo mechanism detects a focus error signal from the reflected light from the optical disk and moves an electromagnetic actuator, to which the objective lens is secured, based on the resulting focus error signal.
As a method of detecting the focus error signal, an astigmatism method is used widely. According to the astigmatism method, when the reflected light from the optical disk is condensed by an optical lens in order to project the light on a photodiode as a light receiving element, the focal length on a first plane of the optical lens including an axis in a focusing direction and the focal length on a second plane of the optical lens perpendicular to the first plane and intersecting with the first plane along the axis in the focusing direction are made different from each other.
With the astigmatism method, the polarity of the focus error signal differs whether the relative position of the objective lens to the optical disk is relatively far or near, and signals having the opposite polarities can be obtained.
Such an optical lens having different focal lengths on first and second planes perpendicular to each other as described above can be obtained, for example, by combining a cylindrical lens and a circular convex lens or by inserting, into a converging optical system for which a circular convex lens for converging light is used, a flat plate in an inclined relationship to an optical axis of the optical system.
However, the conventional method of realizing an optical system whose focal length is different on the first and second planes perpendicular to each other is disadvantageous in that, since it uses a plurality of optical members, a great number of parts are involved and also a great number of locations where adjustment of a part is required are involved, which makes a factor of raising the production cost. The conventional method is disadvantageous also in that the use of a plurality of optical members makes the light path of the converging optical system long, which makes miniaturization of the optical pickup apparatus difficult.